1. Field of the Invention
The present invention relates to an infrared camera, and more particularly, to a thermal infrared camera with a thermal infrared image pickup element.
2. Discussion of the Related Art
Thermal infrared image pickup elements are well known in the art. In such pickup elements, light detectors include a temperature sensor using a thermistor and an infrared radiation absorbing layer including a silicon nitride film that is thermally coupled to the temperature sensor. The infrared radiation absorbing layer generates heat in response to incident infrared radiation, and a temperature change occurs in the temperature sensor. The temperature sensor exhibits a variation in its physical characteristics such as electrical resistance, in response to a change in temperature. The pickup element senses the variation in electrical resistance as a current change or a voltage change, and outputs an electrical signal in response to the infrared radiation.
Depending on the substance used, ferroelectric type, pyroelectric type and capacitance type temperature sensors are used in the art. Thermistors use a substance whose electrical resistivity varies with temperature. For example, vanadium oxide, polysilicon, amorphous silicon and titanium are often used as thermistor substances. Alternatively, pn junctions and Schottky junctions also may be used.
Pyroelectric type detectors are capacitors having metallic electrodes applied to opposite surfaces of the temperature-sensitive pyroelectric (usually ferroelectric) crystal. Modulated radiation incident on the detector gives rise to an alternating temperature change. Accompanying the temperature change is an alternating charge on the external electrodes. An alternating charge on the external electrodes induces current, which is proportional to the rate of temperature change.
Ferroelectric type detectors are very similar to pyroelectric type detectors, but operation of ferroelectric type detectors is based upon the temperature dependence of dielectric permittivity in ferroelectric materials. In both cases, a ferroelectric material, such as BST, is used.
Recently, capacitor type sensors have been proposed in which two electrodes face each other, and a change in a distance between the two electrodes caused by a change in temperature is sensed using a variation in capacitance.
However, in the pickup elements described above, the temperature variation generated in the light detectors is extremely small. For example, if a temperature of a physical object being imaged varies by 1.degree. C., a resulting temperature variation in the light detector is approximately 0.005.degree. C. Temperature resolution of the pickup element can be improved by adding a signal processing circuit following signal read-out in order to detect temperature variations in the physical object with a precision of 0.1.degree. C. (i.e., in order to obtain a temperature resolution of 0.1.degree. C.). However, the light detector must be temperature-controlled to within 0.005.degree. C.
This may be easily understood by considering the fact that it is impossible to determine whether the temperature variation that is read out is due to incident infrared radiation or due to fluctuations in pickup element temperature resulting from changes in ambient temperature. Generally, therefore, an electronic cooling element is bonded to the pickup element, the temperature of the electronic cooling element is measured with a temperature sensor, and feedback control is used to keep the temperature of the pickup element stable.
FIG. 7 is a sectional view of a conventional packaged sensor, and FIG. 8 is an exploded view of the conventional packaged sensor shown in FIG. 7. The conventional packaged sensor includes a main body 205 formed of a metal or a ceramic material. The conventional packaged sensor also includes a cavity (a recessed part) 210, and an infrared radiation transmission window 201. Metal piping 207 is connected to the main body 205. A thermal infrared image pickup element 202 is electrically connected to the outside via wiring 206.
The pickup element 202 is bonded to an electronic cooling element 203 used for temperature control, and is mounted in the cavity 210 together with the electronic cooling element 203. Generally, a Peltier element is used as the electronic cooling element 203. The Peltier element can cool or heat the pickup element 202 to a desired temperature depending on a polarity of applied voltage. A temperature sensor 204 is positioned near the pickup element 202 to measure the temperature of the pickup element 202.
After the electronic cooling element 203 is mounted in the main body 205, the transmission window 201 is bonded to the main body 205 with an adhesive agent 209, and the cavity 210 is evacuated via the metal piping 207 until a vacuum is attained. The metal piping 207 is then sealed by a mechanical constriction 208, completing the packaging of the pickup element 202.
Infrared cameras that use thermal infrared image pickup elements (hereafter referred to as "thermal infrared cameras") are also well known. FIG. 9 is a schematic sectional view of a conventional thermal infrared camera. A filter 229 used to limit the wavelength of incoming light is positioned before a front lens of an image-focusing optical system 221. The filter 229 may include long-pass filters and short-pass filters. The filter 229 passes only infrared radiation of a desired wavelength to the pickup element 202. Furthermore, an optically variable diaphragm 230 behind a rear lens of the image-focusing optical system 221 also is used for image pickup. The diaphragm 230 limits the amount of infrared radiation incident on the pickup element 202.
The pickup element 202 is mounted in a package 231, and the package 231 is mounted in the thermal infrared camera. If necessary, a driving and read-out circuit 223 for the pickup element 202 is installed inside the infrared camera housing 222. The package 231 and the driving and read-out circuit 223 are connected by wiring 224, and the driving and read-out circuit 223 and external circuits (not shown) are connected via wiring 225 and a connecting terminal 226.
In the conventional thermal infrared camera, the image deteriorates when the ambient temperature varies. Thus, the conventional thermal infrared camera is often impractical to use. For example, a thermal infrared camera suitable for indoor use is not suitable for outdoor use because the signal quantity for indoor use is too small and the amount of noise is too large. Moreover, if the same indoor camera is used outdoors at outside air temperatures, image quality is often poor.
As a result of research, the present inventors discovered that it is difficult to alleviate the effects of ambient temperature in the conventional thermal infrared camera merely by maintaining the temperature of the pickup element 202 at a constant value with good precision.
Specifically, the infrared radiation incident on the light detector of the pickup element 202 includes infrared radiation ("signal component") used to obtain the image that passes through the image-focusing optical system 221 used for image pickup as well as infrared radiation ("background light" or noise) that is not needed in order to obtain an image from the thermal infrared camera itself.
The background light is radiated from all the elements within a range viewed by the light detector of the pickup element 202 (e.g., the camera housing 222, a lens barrel (lens housing), the filter 229, and the package 231). These elements radiate infrared radiation due to their respective temperatures and emissivities. If infrared energy radiated by a black body at a certain temperature is taken as 1, the emissivity is a proportion of the infrared energy radiated by a certain physical body at the same temperature as the black body. At least a portion of the infrared energy is incident on the light detector of the pickup element 202 as the background light.
Accordingly, if the ambient air temperature changes, the temperatures of the elements, such as the lens barrel and the camera housing 222 also change in response to the ambient air temperature. Consequently, the background light fluctuates, and the amount of infrared radiation received by the pickup element 202 also fluctuates.
Furthermore, if the ambient air temperature increases, the background light from the Peltier element also increases. When the ambient air temperature changes, a load on the Peltier element also changes, so that a temperature of a secondary side of the Peltier element (i.e., a side opposite the pickup element 202) varies greatly. As a result, the amount of infrared radiation from the secondary side of the Peltier element also varies greatly, and the amount of the background light varies greatly as well.
Thus, in conventional thermal infrared cameras, it has been difficult to reduce the effects of ambient air temperature merely by maintaining the temperature of the pickup element 202 at a constant value with good precision.